Direct in vivo assessment of parathyroid hormone-calcium relationship curve in renal patients

Secondary hyperparathyroidism (SHP) is a well documented finding even in the early stages of chronic renal failure (CRF). A sigmoidal relationship, fitting a four parameter model, links PTH secretion rate and calcium concentration changes. To our knowledge, PTH secretory parameters have only been studied in uremic patients who are in dialysis treatment. As a result of these studies, a possible role for derangement in setpoint values (that is, the serum calcium concentration corresponding to the mid-range value on the sigmoidal curve) has been suggested in the pathogenesis of SHP in CRF. Our study was undertaken to gain insight into the calcium-PTH relationship curve in the first course of CRF and to assess whether a change in any of the secretory parameters is related to the beginning of SHP. We studied 27 male renal patients with a variable degree of renal function (creatinine clearance 12 to 164 ml/min) and 9 control subjects. In all patients and controls the following parameters were evaluated: (1) basal 1,25(OH)(2) vitamin D, 25(OH)vitamin D, calcitonin (CT), intact PTH; (2) GFR by Cr(51)EDTA clearance; (3) the sigmoidal PTH-ionized calcium relation curve, by means of a hypocalcemic stimulating test (Na-2-EDTA 37 mg/kg body weight/2 hr) and a hypercalcemic test (Ca gluconate giving 8 mg/kg of body weight/2 hr of Ca element), performed on two consecutive days. The main results were: (1) the progressive reduction of GFR was accompanied by an increase in the maximum secretory capacity of PTH, without any change in setpoint values; (2) in addition to the already known factors, CT seems to be, in some as yet undefined way, related to PTH hypersecretion in the course of CRF.